1. Field of the Invention
The invention relates to a cooling plate for a furnace in a metallurgical plant. More particularly, it concerns a cooling plate for a blast furnace which cooling plate is made of cast iron and has steel tubes cast therein for receiving the cooling agent and a jacket of one or several layers which sheaths the tubes and is arranged between the tubes and the cast body of the cooling plate.
2. Description of the Prior Art
Cooling plates of this type are arranged in front of the casing or jacket of a furnace in a metallurgical plant, e.g., blast furnace, toward the interior of the furance and protect the casing or jacket against the heat which is discharged to the outside through the refractory furnace lining. For this purpose, vertically extending tubes are cast into the cast-iron cooling plates. These tubes conduct a cooling agent to remove the heat flow which is discharged from the furnace.
To maintain the resistance of the tubes to conduct the cooling agent, the tube material must be protected against the carburizing effect of the high temperature cast iron which surrounds the tube during the casting procedure. For this purpose, it is known to provide between the tubes and the cast body of the cooling plate, a jacket which sheaths the tubes. The jacket may be made of a ceramic material which consists of a mixture of silicon dioxide and dimethyl polysiloxane (German Offenlegungsschrift No. 21 28 827). It is also known to use basic substances, e.g., aluminum oxide, titanium oxide and zirconium oxide.
However, in actual use of such cooling plates whose cast-in steel tubes were provided with a ceramic sheathing, it has been found that this protection is not always sufficient. Due to the sudden heating during casting, cracks will occur in the coating because of the different thermal expansion coefficients of the steel tube and the ceramic layer. At these locations, these cracks will lead to a carburization of the steel tube and, thus, to a reduction of the ductility. This has a particularly disadvantageous effect in the curved tube portions which are under a high mechanical load.
Furthermore, in ceramic layers which are applied by painting or spraying, it is difficult to avoid the presence of very small hollow spaces and air cushions which adversely influence the heat transfer as a result of the formation of air gaps.
However, the removal of the amount of heat applied to the hot side of the cooling plate through the plate into the cooling tubes is the deciding factor with respect to the durability of the plate. When the heat is removed too slowly, the plate may wear prematurely and the steel tube may rupture due to its reduced ductility caused by carburization. The optimum heat removal cannot then be obtained because of the air gap produced by the ceramic sheathing of the tubes, the entrapped air being a poor conductor of heat.